Distortion compensating device for facsimile equipment



March 4, 1952 F. A. HESTER,

DISTORTION COMPENSATING DEVICE FOR FACSIMILE EQUIPMENT 2 SHEETS-SHEET 1 Filed May 29. 1947 TIME T1 N m 5 MN 9 E w T. U mm 1 3 4 5 2 T m I 3 2 EIY 7 B & 6

SOURCE OF SIGNALS UTILIZATION MEANS SOURCE OF SIGNALS FIGS.-

FRANK A. HESTER INVENTOR.

ATTORNEY F. A., HESTER March 4, 1952 DISTORTION COMPENSATING DEVICE FOR FACSIMILE EQUIPMENT Filed May 29. 1947 2 SHEETS-SHEET 2 UZmDOuIm FRANK A. HESTER 2205 m0 uUmDOw Patented Mar. 4, 19 52 N'ITED STATES DISTORTION COMPENSATING DEVICE FOR FACSIMILE EQUIPMENT Franki'A. Hester, New York, N ..Y.',- assignor to Faximile,iInc.', New York, N. 2., a corporation of Delaware 4 Claims.

The present invention concerns electrical-circuits and,vin particular, circuits for improving the definitioninfacsimile recording systems and the like.

Infacsimile systems. and the like, material to be vtransmitted to a-distant point and reproduced is eifectively scanned by a small spot of light'covering anelementalarea: of the copy at a time; The light transmitted through the copy or refiected from the copy is directed to a photo-electric cellby a suitable lens system and variations in the photo-electriccell output due to variationsin density in the elemental scanning area are utilized to actuate a transmittin device. The scanning is usually accomplished in a series of parallel lines successively covering the subject matter. Atlthe reproducing or recording end the received signals are reproduced, for instance, by scanning an electrically sensitive sheet and impressing upon it the signal variations representing the original copy. The scanning spot at the recorder normally has thesamedimensions as, and should move in synchronism with; the scanning spot at the transmitter-in orderto reproducefaithfully the original copy upon the" recording sheet;

In facsimilezsystems and the like, definition may be defined as the number of scanning lines per inch'rof' copy in: one, direction; and as the maximum number ofielemental areas per. inch of copy-in a directioniatfrightangles to the first direction;. which are capableyof. generating a marking signal of. full value Thus, definition isdetermined byrthesize of the elemental area scanned at; any. instant which in turnv is determined by the size of the scanning aperture. The

finest definition theoretically obtainable from a facsimile scanner: is'that where the signalgenerated in a photo-electric cell, as it scansa line of any finite width, is a square wave of constant amplitude'with infinitely small periods of rise and fall. Since an apertureinfinitely small in width. is not possible from a practical standpoint the rise and fall" time willalways have a finite. value. Thus, at a given transmission speed, the larger the aperture; the longer the periods'of rise and fall. Since the band width allowed for transmission of facsimile signals is usually limited,- compromises on transmission speeds and definition commercially acceptable may result in, for example, one hundred five lines per inch at approximately twenty: seven square inches perlminute, orwa maximum elemental frequency of approximately-three thou? sandcyclesiper second; Where the width of-the" area of 'optical density being. scanned is equal to the width ofthe defining aperture the wave shape generated will. be triangular with a base equaliin'time to twice the width of the aperture;

Wider areas will result in a trapezoidal form of full amplitude while 1 narrower areas will result in a trapezoidalform with a maximumamplitude something lesslthanthat required to mark with The definition obtainable in a given facsimile system may be limited by, limiting:the extent of v the spectrum transmitted from scannerto recorder. Forexample, even whenscanning-with a very fine aperture-if no frequencies above three thousand cycles per-second are transmitted; the

rise time and the fall time of the wave maybe made equal-to /6000 of asecond. In this case the: definition, may. besaidto. be limited electronically; and the wave form generated by the smallielemental area will besinusoidal in shape. For purposes of: economy it is usually desirable to transmit asnarrow a band offrequencies' as possible and achieve the required definition.

The greater the speed of facsimile transmis:----

sionv the greater the frequency bandwidth; re-

quired for a given definition, In a Welldesigned;

facsimile system of a givenv definition, the speed of transmission is at the limit determined byv bandwidth, or, expressed another way, the bandwidthis'asnarrow ascan provide the" desired speed and'definition. Spot speed in inches per second multiplied'by the definition in 1 pairs of alternate black and white dots per inch equals the graphic elemental" frequency. A; facsimile system must have a bandwidth atleast equal to the elemental frequency. onlyw equaltothe elemental frequency, the signal correspondingm each pair of black" and white dotscannot: be a cycle'of a square wave;- it is one sine wave cycle of the highest frequency passed-by the system. Similarly-when a larger black spot isscanned; the slopes of the signal cannotbe steeperthanthat of a sine'w-ave-of' the highest frequency passed.

In order to more faithfully reproduce thecopy,

it hasalWays-been thepra'ctice inthe-art to-designa facsimile system with a slow-enoughspeed or a lowenoughdefintion'so'athat thirdand pos-- V sibly fifth harmonics off the elemental frequency If the bandwidth: is

According to the present invention, it is not necessary to pass odd harmonics of the elemental frequency. With a given available bandwidth, the invention permits a greatly increased speed of transmission or a greatly nition. Or, expressed another way, a given speed of transmission and definition can be achieved with a much smaller bandwidth.

According to this invention, the signal waveforms at points corresponding to changes in graphic density are steeper than heretofore could have been achieved with an equivalent bandwidth spectrum. The resulting waveforms, in addition to having steep fronts, have overshoot portions which impart a crisp and pleasing character to the recordings produced therefrom.

In facsimile transmissions involving only variations in optical density from full white to full black, such as, for example, printed materials,

maps, etc., it is necessary to transmit only the fundamental frequency generated by the elemental area, inasmuch as the high frequency components can be restored at the receiving end by amplification and threshold and peak limiting,

thus restoring the square character of the wave;

or, which is the same thing, decreasing the time of the rise and fall of the Wave by electronic means.

In facsimile transmissions involving material which exhibits continuous variation in optical density from full white to full black, such as photographs and the like, if limiting means were employed to increase the definition, the reproduction would obviously be either full white or full black and would not preserve the continuous variations in optical density. In such systems involving the transmission of photographic material it has been common practice to transmit at least the third and often the fifth harmonic of the elemental area frequency so as more nearly to preserve the desired wave shape. This, however, results in the transmission of a spectrum which is either three or five times as great as is required with the black and white system.

It is one object of the present invention to improve detail in a facsimile copy transmitted with a minimum band width and yet substantially preserve the fidelity of photographic reproduction.

Another object is to provide a simple and inexpensive circuit for greatly improving the definition and reproduction from a conventional facsimile signal or the like.

A still further object is to provide circuits for correcting distortion due to the finite aperture at the pick-up end of the system.

A further object is to provide circuits for restoring the wave form lost due to lack of transmission of high frequencies in a facsimile system or the like.

These and other objects of the present invention will be apparent from a detailed description of the invention given in connection with the various figures of the drawings.

According to the present invention the rise and fall time of the signal is decreased to increase the definition of the recorded signal. The present invention may, also, restore partially the harmonics necessary to reproduce a square wave improved defirepresenting a black line at the limits of definition of the system and lost due to a limited band in transmission. The present invention may be practiced in a number of ways as, for instance, by utilizing the characteristics of a capacitance in conjunction with a resistance or inductance to reduce the periods of rise and fall of the signal of any amplitude to a fraction of that normally required.

In the drawings:

Fig. 1 shows various curves useful in explaining the operation of the present invention,

Fig. 2 shows a circuit embodying one form of the present invention.

Fig. 3 shows a circuit embodying a modified form of the present invention.

Fig. 4 shows a circuit embodying a still further modification of the present invention.

Fig. 5 shows curves to further illustrate the operation of the present invention.

Fig. 1 shows a representation of an elemental scanning area I, about to pass over a black line 2, in the direction of the arrow. For purposes of illustration it will be assumed that the size of scanning area I, and the speed with which it is travelling across line 2 is such that it requires /6000 of a second for it to pass over the boundary of the line 2 in the direction of scanning as shown by the arrow. Also in Fig. 1 are two curves illustrating the operation of the present invention in which voltage is plotted vertically against time shown horizontally. Curve A-BC shows the signal current generated as the scanning area passes over line 2. According to the illustration, therefore, the time T1 to T3 and the time from T4 to T6 will be /6000 of a second. Thus, portion A of the curve represents the maximum rate in which the signal can rise due to the limitation of the aperture size and portion C represents themaximum rate at which the signal can decrease. According to the present invention the rate of rise and the rate of fall of the signal is to be actually or apparently increased so that the actual or apparent signal more nearly approximates the theoretical shape which it should have. Since the greatest amount of correction is required where the rate of rise or fall is a maximum the system is designed to increase the rate of rise or fall a maximum for these maximum rates of signal change. According to one embodiment of the present invention, portion A of the curve ABC may be made to jump up suddenly a finite distance and then follow a slope parallel to A to a point considerably above E1 as shown by curve D, and to fall with a sudden initial drop and then a slope parallel to C as shown by curve E. Since E1 represents the maximum density of recording the apparent rise time is shortened from initial value of T1 to T3 a new value of T1 to T2, and the decay time, since the signal cannot go below zero, is apparently decreased from T4 to T6 to a new value of T4 to T5. It will be seen that, although the signal overshoots both in a positive and negative direction, due to the response of the recording sheet the apparent rise and fall time has been decreased to approximately half its initial value. In fact, by causing the signal to overshoot by approximately fifty percent the rise or fall time has been decreased approximately fifty percent. In the right hand portion of Fig. 1 is shown a similar curve G with a rise time T7 to T3 and a decay time of T9 to T10. According to another embodiment of the invention a damped oscillation is produced which both overatomic shoots and reducesithe rise and f'al1 'time1of-'the:.-

signal as-shown by F atthe rising portion of :the-* curve and H at the decay portion ofthe curve. As before, by overshooting by approximately fifty percent the riseandfall timehas been reduced-approximately fifty percent.

Fig. shows another method of illustrating the functioning of the present invention. Curve J is a plot of signalamplitude versus frequency showing the effectof aperture distortion upon the frequency spectrum of the generatedsignal. Curve J falls to zero. when the length of the aperture is equal to one wave length of the .frequency generated due to the scanned copy. Curve J may also be considered to represent the band pass characteristic of the facsimile system.

The speed or definition of a facsimile system may be limited either by the size'of the scanning aperture or by the frequency bandwidth of the system. Previous attempts to overcome the effect in systems of this kind have been along the line of flattening curve J, bringing it up at the high frequency end to have a characteristic somewhat as shown at K. According to the present invention, instead of trying to correct the frequency response as above described, a much smaller frequency range is utilized, and by giving the curve a sharply rising characteristic as shown at L the improvements herein described may be realized. Curve L rises sharply to a point considerably'above any point which would obtain to afiat frequency characteristic and then falls at a point considerably short of the theoretical zero point of curve J. The maximum response point, for example, may be made to come at approximately one-half of the original cut-off frequency. The maximum response point should be at a frequency in cycles per second equal to the graphic elemental frequency which is defined in the art as the spot speed in inches per second multiplied by the definition in pairs of alternate black and White dots per inch. According to this invention, as is apparent from the drawing, odd harmonics of the elemental frequency are not relied on to provide speed or definition. The elemental frequency is substantially as great as the effective bandwidth of the sys-. tem.

Fig. 2 shows a circuit embodying one form of the present invention. Between a source of signal 6 having output terminals '1 and 8 and a utilization means 9 having input terminals ill and II is connected a differentiating circuit consisting of resistor 3 and shunting capacitor l in series with resistor 5. The input signal E2 from source 6 is applied across a series combination and the output signal E3 is taken off across resistor 5. If the value of resistor 5 is made low compared to the reactance of capacitor 4 at the upper frequency limit which it is desired to compensate, a curve as shown at D in Fig. 1 will be obtained. The vertical portions of curve D, the overshoot and hence the apparent increase in slope of the resulting curve may be controlled by the relative values of capacitor 4 and resistor 5. The amplitude of the fiat portion B of the curve is controlled by the relative values of resistors 3 and 5.

Fig. 3 shows another embodiment of the present invention which is somewhat more flexible than the circuit shown in Fig. 2. Signals derived from the source of signal 6 which it is desired to compensate, as described above, are applied to two tubes I7 and I3 having their plates and [6 connected in parallel across the 6: common load. 21 and feeding a utilization means 9. gridi'l5=:of tube :l3 whilethe differentiatedportiondsapplied to grid- 19 oftube ll throughseries capacitor 23 and. across an adjustable adjustment may be made to compensate the aperture characteristic and subject matter characteristics in any givensystem or at any given time of transmission.

Fig. 4-shows a still further modification of the present invention in which facsimile signals from a source-6 are to be compensated and applied to utiliza-tion'means 49-. In this-case-the damped oscillation type-of compensation as shown at 1- andH of Fig. 1 is utilized. One convenient manner in'which this damped oscillation may be generated is by means of a series circuit consisting of resistor 35, inductor 35 and capacitor 3? connected in series as a load for plate 32 of amplifier tube 28. Resistor BQ-is made large and serves chiefly to supply plate voltage from plate battery Mwtoplate 32. Resistor 35 acts as'a damping resistor and is chosen to give a low Q to the series circuit. When signals of the type above described are impressed upon grid 39 of tube 28 a damped oscillation is set up in the plate circuit and the voltage appearing across capacitor Bl will have a form as shown in F and H of Fig. 1. This voltage is applied to grid M of amplifier-tube "32 through a suitable means such as coupling capacitor 39 and from plate 66 of amplifier tube 42 to utilization means d9 through suitable means such as coupling capacitor 23. The values of inductor 36 and capacitor 37 are chosen to resonate at a frequency approximately one-half the cutoff frequency of curve J in Fig. 5 and the resulting frequency response will be similar to that shown by curve L in Fig. 5.

It is thus apparent that according to the present invention the real and apparent detail of a facsimile system or the like may be greatly improved in a simple and effective manner and without increasing the frequency range required. The amount of improvement may be readily controlled to meet various requirements. The real and apparent improvement in sharpness of contrast is accomplished without sacrifice to fidelity in the transmission of half tone copy, since the maximum increase in signal slope is only produced wherever the original signal has maximum slope as at a full white to full black boundary.

While several embodiments of the present invention have been shown and described, other variations will be apparent to those skilled in the art as within the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. In a recording amplifier of a facsimile system, a source of a facsimile signal varying in amplitude in accordance with variations in graphic density, a waveform changing circuit receptive to the signal from said source and operative responsively to each change in amplitude of said signal from a first level to a second level to generate a wave extending momentarily beyond said second level and then returning to said second level, and facsimile recording means receptive to the output of the waveform changing The complete signal is applied to.

circuit, whereby changes in graphic density are emphasized in the recording.

2. In the recording amplifier of a facsimile system having an elemental frequency 1, a source of a facsimile signal varying in amplitude in accordance with variations in graphic density subject to the limitation of not including frequency components substantially in excess of frequency f, a waveform changing circuit receptive to said signal and operative to generate a waveform differing from the one received in that the slopes of the waveform are steeper and in that momentary overshoots accompany changes in amplitude, and facsimile recording means receptive to the output of the waveform changing circuit, whereby the frequency bandwidth channel of the facsimile system may be economized to exclude odd harmonics of the frequency f.

3. In facsimile communications, the method of recording which comprises the steps of receiving a facsimile signal modulated in amplitude in accordance with graphic density subject to the limitation of not including frequency components substantially greater than a frequency of 1 cycles per second, changing the waveform to the extent of steepening slopes and momentarily overshooting changes from one level to another, and applying the changed waveform to recording apparatus, whereby recordings may be made having a quality represented by an elemental frequency equal to 3 cycles per second where elemental frequency is equal to the spot speed in inches per second multiplied by the definition in pairs of alternate black and white dots per inch.

4. In the recording amplifier of a halftone facsimile system having a maximum effective elemental frequency 1 determined by aperture distortion or bandwidth limitations, or the like,

the combination of a source of facsimile signal varying in amplitude in accordance with variations in graphic density subject to the limitation of not including frequency components substantially in excess of frequency f, a waveform changing circuit receptive to said signal comprising a resistor, an inductor and a capacitor connected in series, the values being selected to resonate substantially at frequency f, means for taking an output from across said capacitor, the output waveform differing from the input waveform to the waveform changing circuit in that the slopes are steeper and in that momentary overshoots accompany changes in amplitude, and facsimile recording means receptive to said output, whereby the frequency bandwidth channel of the facsimile system may be economized to exclude odd harmonics of the elemental frequency f.

FRANK A. HESTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,516,518 Carson Nov. 25, 1924 1,717,624 Ranger June 18, 1929 2,167,136 Wheeler July 25, 1939 2,183,203 Poch Dec. 12, 1939 2,202,361 Verbeek et a1 May 28, 1940 2,240,605 Bingley May 6, 1941 2,301,199 Bruce et a1. Nov. 10, 1942 2,332,919 Kleen Oct. 26, 1943 2,340,317 Finch Feb. 1, 1944 2,380,482 Tribble July 31, 1945 2,384,263 Schlesinger Sept. 4, 1945 2,453,081 Sziklai Nov. 2, 1948 

